This invention relates to a photodetector for use in reproducing information recorded on an optical recording medium.
Upon recording of data from a music or image source on an optical disc recording medium, the data are converted into a digital signal having logic "0" and "1" levels. According to the logic level, pits are formed to provide continuous spiral recording track in the optical disc recording medium. An optical pickup device is used to reproduce the recorded data by radiating a light beam from a light source such as a semiconductor laser through an objective lens which focuses the light beam on the optical disc recording medium and sensing the light beam reflected from the optical recording medium to determine the presence of pits.
According to an astigmatic method, the optical pickup device employs a photodetector placed at a position intermediate between the two astigmatic points of the light beam which has reflected from the optical disc recording medium and passed through a cylindrical lens. The photodetector has a center photosensitive section juxtaposed in a spaced relationship between two side photosensitive sections. The center photosensitive section is divided by a cross-shaped electric and optical insulator into four portions each being formed by a independent photodiode. The shape and position of the reflected light spot formed on the center photosensitive section is dependent upon the distance between the optical disc recording medium and the objective lens. A circular light spot is formed on the center photosensitive section when the objective lens is positioned in focus with respect to the optical disc recording medium. An eliptical light spot is formed on the center photosensitive section when the objective lens is at a shorter or longer distance from the optical disc recording medium. Formation of a light spot on the center photosensitive section causes currents to be derived from the respective divided portions of the center photosensitive section. The derived currents are used to reproduce the recorded data and also to detect an focusing error for a focusing servo control. The side photosensitive sections, each of which is formed by a photodiode, detect respective side beams for a tracking servo control.
One problem occurs in eliminating jitter of the main of RF signal transmitted from such a conventional photodetector in order to improve the reading performance of the optical pickup device. Although attempts have been made to eliminate such jitter by increasing the machining accuracy with which the optical disc recording medium is produced, the results were not satisfactory in some applications.